Triple Cross / HO HO – Interlocking puzzle by Stewart Coffin (STC #79)

Triple Cross / HO HO

Interlocking puzzle by Stewart Coffin (STC #79)

Assemble 12 identical pieces into the cross shape shown in the cover photo (Triple Cross). For added challenge, assemble 14 such pieces into the box shape in the second photo (HO HO).

Triple Cross is a relatively neglected design by Stewart Coffin. Consisting of twelve identical pieces (or fourteen for the HO HO variant), the figures are more difficult to assemble than one might expect. Coffin wrote:

“The 12 identical pieces of Triple Cross or 14 pieces of HO HO assemble in the familiar Square Knot #9 and Plus 2 #57 configurations but with a completely different type of joint. I made one model in 1973 as a prototype for manufacture in plastic, but of course that never happened …. It is probably impractical to make these in wood, which is unfortunate because I think this scheme might have much potential as a set of puzzles, a pastime, a construction kit, or an educational toy. Longer pieces with more notches could add to the possibilities. Can you see how the 14-piece version got the name HO HO?”

Triple Cross is also notable for being the subject of one of the few (perhaps only?) references to 3D printing in Coffin’s writings: a copy was sent to him “by Steve Nicholls and printed by [Nicholls] in ABS plastic. It fits with a degree of precision that I can’t imagine achieving in wood.”

Well, I’m pleased to make this model available, so that anyone can print their own copy to a similar degree of precision. (There’s no need to follow Nicholls and print in ABS, as PLA works just as well.) In case you want to experiment with “longer pieces with more notches,” as suggested by Coffin, I’ve included some variant pieces as well. Moreover, the puzzle is modeled with OpenSCAD, and you can use the .scad file to generate pieces of any length. As far as I know, such possibilities have never really been investigated, so there is perhaps much to discover here.

Printing Instructions

Print 12 copies of coffin.triple-cross.piece.stl, or 14 if you also want to be able to construct HO HO. To get the color scheme shown in the photo, print 4 copies in each of three filament colors (or 4, 4, and 6 for HO HO). You will need to print with “supports on build plate only” enabled. For best results, set the support overhang to something less than 45 degrees; supports are needed only to span a roughly 1 mm gap along the base, and aren’t necessary along the upward slopes of the pieces.

coffin.triple-cross.piece-length-4.stl and coffin.triple-cross.piece-length-5.stl offer pieces of length 4 and 5, respectively. Since those pieces are quite a bit larger, and many of them are needed to experiment with larger assemblies, I’ve also included smaller versions of each piece size (16 mm unit size versus 20 mm).

The Printable Puzzle Project

The Printable Puzzle Project aims to make available high-quality open-source models of many puzzle designs. All of our models are posted with the generous permission of their designers and are licensed for non-commercial use only. Anyone may print copies for their own personal use, but selling or otherwise monetizing them is not permitted, and puzzle designers retain all rights as copyright holders of their work.

Our puzzles are modeled using the open-source puzzlecad library. The .scad file is included with this model in case you want to modify any of its design parameters; more information on how to do this can be found in the PPP Puzzle Modeling tutorial.

About the Designer

Stewart Coffin has been called “the most outstanding designer and maker of interlocking puzzles that the world has ever seen”. He is credited with over four hundred designs, has pioneered numerous ingenious puzzle forms, and has written extensively about puzzle design and craftsmanship. More information on Coffin, along with many other printable models of his designs, can be found on the Stewart Coffin Puzzles overview page.

Happy puzzling!